Fire detector circuit arranged to prevent false warning



Dec. 15, 1964 J. E. LINDBERG 3,151,364

FIRE DETECTOR CIRCUIT ARRANGED TO PREVENT FALSE WARNING Filed May 21,1962 2 Sheets-Sheet 1 36 f l I I, ll

, l I I FIG. 2

JOHN E. LINDBERG INVENTOR.

@Uzw- Dec. 15, 1964 Filed May 21, 1962 J. E. LINDBERG FIRE DETECTORCIRCUIT ARRANGED TO PREVENT FALSE WARNING e. mt

2 Sheets-Sheet 2 JOHN E. LINDBERG INVENTOR.

United States Patent 3,161,864 FIRE DETECTOR CIRCUIT ARRANGED TO PREVENTFALSE WARNING John E. Lindberg, 3296 Springhill Road, Lafayette, Calif.Filed May 21, 196 2, Ser, No. 196,303 Claims. (Cl. 340214) Thisinvention relates to improvements in warning or alarm circuits,especially to those used for the detection of fires in airplanes andelsewhere.

Fire-detection circuits andrsome other types of warning circuitsheretofore in use have been prone to give false alarms. Recentstatistics ihdicate, for example, that on the commercial airlines in theUnited States alone there are, on the average, two false fire warhingsper day. During one period of time for-orie type of aircraft, there wereno more than two engine fires "reported (and orie of them not confirmed)while thirty-seven engines were reported shut down in flight andeighty-five ground delays or interruptions were reported, all due tofalse fire wa'rnirigs. Each false warning in flight is liable to cause afatal crash, and several of these have occurred each year in recentyears. Even when the plane and passengers are saved, each false firewarning in flight requires immediate remedial action, such as dumpingexcess fuel dowh to the allowable landing weight and landing as soon aspossible. The resultant disruption of the schedule, the cost of landing,taking off again, arid obtaining clearances, and other expensesresulting from this false warning have been estimated to add :more than$20,000 on the averageper false fire warning on large commercial planes.

An important object of the invention is. to provide a fire detectioncircuit far less liable to give false fire warnings than thoseheretofore in use.

An important cause of false warnings in fire-detection systems has beenshort circuits across the electrical portion of the fire detector, andit is an object of this invention to prevent such short circuits, sothat they become next to impossible.

Many current fire detectors give false warnings when a portion of thecircuit is grounded, and another object of the invention is to provide afire-detection circuit that will almost never give false warnings evenwhen the circuit is grounded. I

Other objects of the invention will appear from the followingdescription of some preferred embodiments.

In the drawings:

FIG. 1 is a fire detection circuit diagram embodying the principles ofthe invention. The fire detector sensor is broken in order to conservespace.

FIG. 2 is a view in elevation and in section of a portion of a tiredetection unit embodying a modified form of the invention. a h l h FIG.3 is a circuit diagram of another modif ed form of the inventiori, inwhich a battery is rechargeable.

FIG. 4 is a circuit diagram of still another modified form of theinvention in which the battery of FIG. 2 is rechargeable. p V

FIG 5 is a circuit diagram of a modified former the invention generallylike FIG. '1 "showing the use of alternating curreht. ,g V p A key pointof some forms of this invention is the use of a battery as thepowersource and, more important, its location closely adjacent the firedetector responder. FIG. 1 shows a circuit using a fire detector sensorand responder 11 like those described in application Serial No.

815,406 filed May 25, 1959 now Patent No. 3,122,728,

issued February 25, 1964. For those unacquaihted with that application,it may be said that the sensor. 10 contains a gas or a gas-emittingmaterial that when heated increases the ressure inside the sensor andtransmits that ressure 3,16 1,864 Patented Dec. 15, 1964 to theresponder 11, a pressure-actuated switch that, when actuated, closes theotherwise open circuit. The sensor 10 of Serial No. 815,406 is notelectric in its function, arid therefore the actuation under operationalconditions of the responder 11 does not depend on any electric circuit.Actuation of the responder is indicated by a wariiiiig device 12,comprising, for example, two lamps 13 and-14 and a horn 15 in parallel.

A key feature of this invention is the location of a battery 16, as apower supply between the responder ll-aiid the warning device 12, and,in fact, the closer to the re, sponder 11 the better. In FIG. 1, thereis a very short line 17 connecting the battery 16 to the responder 11,and a much longer line 18 connecting the battery 16 to the warningdevice 12. Return lines 19 arid 20 connect the warning device 12 to theresponder 11. v

For test purposes, a switch 21 and line 22 connect a larger test battery23 to the sensor 10, returning through the line 20, heating the sensor10 and actuating the responder 11 when the testing switch 21 is closed.For normal use, the test circuit may be disregarded, for it is not acause of false warning, cannot itself actuate the warning device 12, andoperates only when the switch 21 is elosed.

The key featurepf locating the battery 16 for the warning circuitclo's'e to the responder 11 is quite important, for no combination ofgrounds, short circuits, or wiring failures which does not include theshortlead 17 can give a false warning, for unless the line 17 is shortedto the other side of the battery 16, the battery cannot cause the lamps13 or 14 to light or the horn 15 to sound. This happens when theresponder 11 is actuated but hot otherwise. Thus, anyof lines '18, 19,or 20 is grounded, or if all of lines 18, 19, and 20 are grounded orshort circuited to each other, there will still be no false warning solong as the short line 17 isnot grounded or connected to one of thelines 19 or 20. Of course, the line 17 can even be grounded withoutproducing a false warn;

ing so long as no other line is grounded, but since the line 17 isshort, special care is easily taken to prevent its grounding. H

A special instance of such care is shown in FIG. 2. Here, the responder11 is surrounded by a housihg 25. At the lower end a snap ring 26 holdsin a flanged tubular insulator 27, and an insulator disc 28 retains aniiisulating sleeve 30 inside the upper portion of the housing 25 abovethe responder 11; vA battery 16, prefe'rablyof the nickel-cadmium type,is located directly on top hf .a responder housing member 31 with theconductor 17 wholly inside the housing 31. The battery 16, has aterminal 32, against which a contact member 33 is held by a spring 34that rigages the disc 28. The member 33 has a stud 35 that extendsoutside the disc 28, and the wire 18 is connected to it, as by a plugthreaded onto the housing 25. The battery 16 can even be inside theresponder housing 36, by forming the housing fifi large'r in order toinclude it. The length of the lead 17 is about the minimumland is alsoalmost fully protected from grounding.

Providedthat the battery 16 is of rechargeable type, such as arechargeable nickel-cadmium cell or a wet cell, it may be recharged asshown in FIG. 9 Here a source of power 40, such as a generator, isconnected to ground through an upper resistor 41 of a suitable highvalue, a central resistor 42 adapted to" a potential drop correspondingto the voltage of the battery, and a lower resistor 43 of high value.Thus, for a 28-volt power source 40, the resistors 41 and 43 mayeach be500,000 :ohms,

and the resistor 42 may have a value of 10,000Yohms. The resistor 42 isconnectedac'ross thebattery- 16 by. leads 44 and 45. The battery 16operates when the generator I 40 is not used andis recharged whenthegenerator40 is used. Preferably, one of the leads 44, 45 contains ablocking rectifier 44a to prevent discharge of the battery 16 when thepower source 4%) is discharged or disconnected.

When the battery 16 is inside the responder 11, as in FIG. 4, it maystill be recharged at testing time, by connecting a charging supply 46to the line 19 by a line 47 and to a switch contact 48 by a line 49. Theline 19 is thus divided into segments 50, 51 and 52. When a switch 53 isconnected to a terminal 54, all three segments 50, 51 and 52 are in thecircuit with the battery 16, depending on actuation of the responder. Atest switch 55 connects a test source of power 56 into a circuit of thesensor, the line 22 and the segment 20. When the switch 55 is thrown totest, the power source 56 heats the sensor 10 and causes the responder11 to close the circuit. Then the switch 53 can be thrown and thecharging supply 46 supplies current that acts to charge the batteryduring testing. By separate operation of the switch 55 when the switch53 is in its normal, non-charging position, the operability of thebattery 16 can be tested.

FIG. shows a circuit generally like that of FIG. 1, with the samenumbers generally used, except that a transformer 6%) is used in placeof the battery 16. This, of course, has some disadvantages, in that thelocation of the transformer 60 causes problems as compared withbatteries, but it will be noted that grounding of the transformerprimary 61 or of its leads, does not elfect actuation of the signalmeans 12,- and, of course, again, grounding of the lines 19 and 20 doesnot actuate the signal means 12. Therefore, again the likelihood of afalse fire warning is reduced.

To those skilled in the art to which this invention relates, manychanges in construction and widely differing embodiments andapplications of the invention will suggest themselves without departingfrom the spirit and scope of the invention. The disclosures and thedescription herein are purely illustrative and are not intended to be inany sense limiting.

' I claim:

1. A critical-temperature-detection system comprising an imperforateelectrically conductive enclosure of extended length;

heat-to-pressure transducing means for substantially increasing thepressure within said enclosure when said:

enclosure is externally heated; pressure-actuated switch means actuatedby the pressure in said enclosure;

an electrical warning circuit including, in series, signal means, afirst source of electrical current, said switch means, and a return linefrom said switch means to said signal means, so that when said firstswitch means is closed by a pressure increase in said enclosure itcauses said warning circuitto actuate said signal means; and

a test circuit including, in series, a second source of electricalcurrent, an external switch, said conductive enclosure, and said returnline to said second source so that said transducing means can beelectrically heated to cause closure of said pressure-actuated switchmeans and therefore actuation of said signal means, 7

said return line being connected to both said conductive enclosure andto one side of said pressure-actuated switch means, and said firstsource being located between the other side of said. pressure-actuated 7switch means and said signal means,

so that grounding of said enclosure or said return line cannot result ina false warning by actuation of said signal means, whereas actualclosing ofsaid pressureactuated switch means does result in actuation ofI said signal means,

.2. The system of claim 1 wherein said first source of electricalcurrent comprises a battery located physically closely adjacent saidswitch means.

3. The system of claim 1 wherein said first source of electrical currentcomprises a transformer.

4. A critical-temperature-detection system comprising an imperforateelectrically conductive enclosure of extended length;

heat-to-pressure transducing means for substantially increasing thepressure within said enclosure when said enclosure is externally heated;

a housing having a first end connected to said enclosure and a secondend;

pressure-actuated switch means in said housing actuated by the pressurein said enclosure and having a first side connected to said enclosureand a second side;

a source of electrical current connected to said second side of saidswitch means and physically located closely adjacent said second end ofsaid housing;

signal means connected to said second side of said switch means throughsaid source;

and return means from said first side of said switch means and saidenclosure to said source signal means so that when said switch means isclosed by a pressure increase in said enclosure it causes said circuitto actuate said signal means and so that grounding of said enclosure andsaid return means does not cause actuation of said signal means; and

a test circuit including means for sending current through saidenclosure and at least part of said return means for electricallyheating said transducing means and thereby causing closure of saidswitch means and therefore actuation of said signal means.

5. A critical-temperature detection system for use where there is adetection zone in one location and a signal zone in a different locationdistant from the detection zone, comprising:

indicator means at said signal zone,

a housing at said detection zone, having a switch chamber,

a battery inside said housing and insulated therefrom,

electrical conduit means external of said housing, said conduit meansconnecting said indicator means to one terminal of said battery,

pressure-actuated switch means inside said chamber and including astationary electrode and a movable switch member, said stationaryelectrode being in direct contact with a second terminal of saidbattery,

an imperforate stationary enclosure of extended length in said detectionzone, connected to said chamber and otherwise external of said housing,I

heat-to-pressure transducing means in said enclosure for substantiallyincreasing the pressure within said enclosure when said enclosure isexternally heated,

return electrical conduit means external of said housing and connectingsaid indicator means to said movable switch member so that said movableswitch member is operated by a pressure increase in said enclosuremoving said movable switch member against said stationary electrode,said battery then actuating said indicator means,

the direct engagement of said battery by said stationary electrodeinside said housing preventing false alarms, since said indicator meanscannot be actuated by grounding of either or both of said conduit means.

References Cited in the file of this patent UNITED STATES PATENTS

1. A CRITICAL-TEMPERATURE-DETECTION SYSTEM COMPRISING AN IMPERFORATEELECTRICALLY CONDUCTIVE ENCLOSURE OF EXTENDED LENGTH; HEAT-TO-PRESSURETRANSDUCING MEANS FOR SUBSTANTIALLY INCREASING THE PRESSURE WITHIN SAIDENCLOSURE WHEN SAID ENCLOSURE IS EXTERNALLY HEATED; PRESSURE-ACTUATEDSWITCH MEANS ACTUATED BY THE PRES SURE IN SAID ENCLOSURE; AN ELECTRICALWARNING CIRCUIT INCLUDING, IN SERIES, SIGNAL MEANS, A FIRST SOURCE OFELECTRICAL CURRENT, SAID SWITCH MEANS, AND A RETURN LINE FROM SAIDSWITCH MEANS TO SAID SIGNAL MEANS, SO THAT WHEN SAID FIRST SWITCH MEANSIS CLOSED BY A PRESSURE INCREASE IN SAID ENCLOSURE IT CAUSES SAIDWARNING CIRCUIT TO ACTUATE SAID SIGNAL MEANS; AND A TEST CIRCUITINCLUDING, IN SERIES, A SECOND SOURCE OF ELECTRICAL CURRENT, AN EXTERNALSWITCH, SAID CONDUCTIVE ENCLOSURE, AND SAID RETURN LINE TO SAID SECONDSOURCE SO THAT SAID TRANSDUCING MEANS CAN BE ELECTRICALLY HEATED TOCAUSE CLOSURE OF SAID PRESSURE-ACTUATED SWITCH MEANS AND THEREFOREACTUATION OF SAID SIGNAL MEANS, SAID RETURN LINE BEING CONNECTED TO BOTHSAID CONDUCTIVE ENCLOSURE AND TO ONE SIDE OF SAID PRESSURE-ACTUATEDSWITCH MEANS, AND SAID FIRST SOURCE BEING LOCATED BETWEEN THE OTHER SIDEOF SAID PRESSURE-ACTUATED SWITCH MEANS AND SAID SIGNAL MEANS, SO THATGROUNDING OF SAID ENCLOSURE OR SAID RETURN LINE CANNOT RESULT IN A FALSEWARNING BY ACTUATION OF SAID SIGNAL MEANS, WHEREAS ACTUAL CLOSING OFSAID PRESSUREACTUATED SWITCH MEANS DOES RESULT IN ACTUATION OF SAIDSIGNAL MEANS.